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What is Vapor Recovery
Vapor recovery is a process used to contain and recover environmentally hazardous and/or valuable vapors. In industrial applications, the process consists of a closed venting system from the storage tank ullage space to a vapor recovery unit, or VRU, which will recover the vapors for return to the process being used.
Vapor recovery units have become commonplace in the oil and gas industry as a way of recovering and converting gas vapor into a viable product. Ejector Vapor Recovery Units are a technological advancement in the vapor recovery field. These units use a closed loop system for vapor recovery and to reduce methane and VOC emissions.
Why is it Important
With the adoption of strict emissions laws at state and federal levels, it has become increasingly difficult for industrial factories and oil refineries to stay within the legal emissions parameters. Factories and refineries produce massive amounts of hydrocarbon pollutants which far exceed federal limits.
Beyond emission standards compliance, companies need a way to capture valuable chemical compounds found within gases that their factories release. The amount of hydrocarbons withing untreated factory emissions is staggering. Especially considering that these chemicals can be used to turn a profit.
Vapor recovery plays a crucial role in environmental protection as well as making more use from oil, natural gas, and other chemicals. Without vapor recovery, the gases released into the atmosphere from refineries and industrial chemical plants would be laden with environmentally detrimental hydrocarbons.
The process of vapor recovery drastically reduces the amount of hydrocarbons released into the atmosphere by collecting them for further refinement into other useful compounds and products. By using vapor recovery technology, companies make more profits while causing less harm to the environment.
How it Works
There are three main types of industrial vapor recovery methods in use today:
- Activated Carbon Adsorption Vapor Recovery
- Refrigeration Condensation Vapor Recovery
- Lean Oil Absorption Vapor Recovery
Activated Carbon Adsorption Vapor Recovery
Activated carbon is extremely porous and has a quite large surface area. This makes it a perfect tool for adsorbing hydrocarbons from within collected vapors. Carbon’s adsorption ability is further enhanced by higher pressure as well as lower temperatures. Adsorption continues until the carbon is completely saturated.
Once the carbon is filled to capacity, the hydrocarbons are removed by a combination of decreasing pressure, reducing hydrocarbon concentration, and increasing the carbon bed’s temperature.
Types of Activated Carbon Vapor Recovery
There are two types of activated carbon vapor recovery systems.
- Pressure Swing Adsorption Activated Carbon Technology
- Thermally Regenerated Activated Carbon Technology
Pressure Swing Adsorption (PSA)
The pressure swing vapor recovery system uses at least two carbon vessels in order to maintain uninterrupted service. Vapors are dispersed through activated carbon beds in one of the vessels, and clean air exits through vents. As the active carbon vessel adsorbs hydrocarbons the second previously used vessel is in the regeneration process. The hydrocarbon is removed from the activated carbon using a mild vacuum. Hydrocarbon vapors and liquids collected in the vacuum system then make their way into the separator vessel.
Usually, a liquid ring vacuum pump or a liquid ring pump in conjunction with a positive displacement pump is used during the regeneration process. The separator vessel separates the seal fluid from the vapor and condenses the hydrocarbon liquid. Cooled seal fluid is then returned to the vacuum system. Non-condensed vapors are routed to either a packed absorber column, direct contact condenser, or a refrigerated condenser. The resulting hydrocarbon liquid is collected and pumped to a liquid storage station. Uncondensed hydrocarbons are then recycled to the active vessel or a vapor holder.
Thermally Regenerated Activated Carbon Technology
This type of carbon vapor recovery system uses a minimum of two carbon beds, and is typically used in low vapor applications.
As mentioned before, a dual bed system uses one vessel to adsorb vapors, while the other bed condenses them. When the bed is completely saturated, steam or hot inert gas is used to remove the hydrocarbons. These hydrocarbons then move on to the condensation process.
Refrigeration Condensation Vapor Recovery
This vapor recovery system was one of the first to be used. On paper, refrigeration condensation is simple – chill the vapor sufficiently to allow heavier hydrocarbon molecules to condense out of the steam. In practice, however, it is slightly more complicated.
Complications arise from the water that is almost always present in the vapor. Because of this moisture, if the vapor’s temperature drops below freezing, the coils can get plugged by a sort of water/hydrocarbon slush reducing the system’s effectiveness.
To counteract this, the vapor is chilled in stages.
Stage One: In this stage the vapor is cooled to just above freezing. This condenses most of the moisture out of the vapor. The water is then collected and the small amount of hydrocarbons remaining in the water are decanted. The waste water is then sent to a treatment facility.
Stages Two and Three: The second stage sees the vapor temperature reduced to a level that brings the hydrocarbon concentration to within the highest emissions limit. In stage three, vapor temperature is further reduced, removing more hydrocarbons to meet stricter emissions requirements.
Lean Oil Absorption Vapor Recovery
Lean Oil Absorption is another early method of recovering vapors. This vapor recovery system is fairly straightforward. Lean oil is forced into contact with with the vapor. This is most often done in a column whereby the vapor rises through the column pushing against the flow of the liquid making its way down the column.
Lean oil gets its name from the fact that it is devoid of hydrocarbons, or lean. When the lean oil and vapor are brought together, the hydrocarbons in the vapor are absorbed by the lean oil. If this is done for a long enough time, the liquid and the vapor will equalize and the liquid will no longer be able to absorb any more hydrocarbons. At this point the liquid is considered rich.
At this point the liquid must be regenerated. The hydrocarbons are removed and recovered from the liquid using heat. Afterwards, the liquid can be reused.